Unequal-Match Theory Of Group-Velocity In Process Of Broadband Third-Harmonic Generation

As the key point of systems like Inertial Confinement Fusion (ICF) and ultra-short UV light source, performance (e.g., conversion efficiency and bandwidth) of broadband third-harmonic generation (THG) directly determines the success of the whole system. However, due to great group-velocity mismatches between coupling waves, it is difficult to balance between these two important parameters (i.e., conversion efficiency and bandwidth) in the THG process, which severely restrict the overall performance of systems like ICF and ultra-short UV light source.With the purpose of uncovering the group-velocity-matching relationship of broadband third-harmonic generation, the effect of group-velocity mismatch in the process of broadband THG is investigated intensely here. The main contents and results of this dissertation include:First, the broadband THG process of ultra-short pulses is studied theoretically and numerically; in the case of small-signal approximation, the explicit group-velocity-matching criteria for the THG process of ultra-short pulses is derived for the first time, which is , where the parameter x is the square ratio of pulse-duration between fundamental and second-harmonic pulses . This formula is called the un-equal group-velocity-matching relationship, and accordingly a general concept for the matching of group-velocity,"un-equal group-velocity-matching", is proposed for the first time.Second, the broadband THG process with sinusoidal phase-modulated pulses input is studied theoretically and numerically; in the case of small-signal approximation, the explicit group-velocity-matching criteria for THG process is derived for the first time, which is , which is amazingly consistent with the one for ultrashort pulses. The only difference is the physical meaning of the parameter x , here x is the ratio of modulation depth between second-harmonic and fundamental pulses, i.e., 2Third, effects of the group-velocity mismatch (GVM) between FH and SH pulses on THG conversion properties of ultra-short pulses are investigated theoretically. Explicit expressions of amplitude of TH pulse in terms of group-velocities of FH and SH pulses are derived analytically under the plane-wave and pump un-depletion approximations. Both theoretical and numerical results show that THG conversion efficiency exponentially decreases along with the square of GVM between FH and SH pulses. The larger the GVM is, and the lower conversion will be.Fourth, based on the dispersion characteristics of crystal, the relations between crystal retracing point and group-velocity-matching are studied in theory. Results show that at the retracing point of crystals, the three-wave group-velocity relationship turns to be which is only a special case of when the parameter x is equal to 2. Simulation results indicate that: when the crystal work at the retracing point of THG process, for ultra-short pulses input case, we can get the highest conversion efficiency and maximum conversion bandwidth; for sinusoidal phase-modulated pulses input case, the FM-to-AM effect can be suppressed effectively. Fifth, techniques, like making good use of electro-optical effect of crystals, are proposed to realize the"un-equal group-velocity-matching"in the process of broadband THG experimentally. Results reveal that tunable THG retracing point of crystals can be obtained by applying external electric field on crystals properly. Meanwhile, fast-wave delay technology after satisfying the"un-equal group-velocity-matching"is proposed to further improve the third-harmonic conversion efficiency of ultra-short pulses, by controlling the into-crystals time difference between FH and SH pulses precisely.In conclusion, my work uncovers some basic and intrinsic laws of group-velocity -matching in broadband third-harmonic-generation process, which plays an important role in solving the group-velocity-mismatch problem in that process. The obtained results provide a theoretical guidance in compensating for the severe group-velocity-mismatch, improving the performance of existing THG conversion system of ICF and ultra-short UV light source, as well as in designing or developing next generation of THG conversion systems with broader bandwidth and higher conversion efficiency.